What is the primary function of the 'cv' tag in an ARC-Seal header?

The 'cv' (chain validation) tag indicates whether the previous ARC chain was successfully validated when the current ARC-Seal was added. It signals the integrity of the email's authentication history to receiving mail servers.

What are the possible values for the 'cv' tag and what do they mean?

The 'cv' tag can have three values: cv=none (no prior ARC chain), cv=pass (previous chain validated), or cv=fail (previous chain failed validation). These values help determine the trustworthiness of the email's authentication journey.

How does 'cv=fail' impact email deliverability?

A 'cv=fail' indicates a break in the ARC chain's integrity, which can significantly reduce the email's trustworthiness. This often leads to messages being flagged by spam filters, resulting in quarantine or rejection , even if the original sender had proper authentication.

Why is it important to monitor the 'cv' tag?

Monitoring the 'cv' tag is crucial for understanding how your emails are being handled by intermediaries and for diagnosing deliverability issues . Consistent 'cv=pass' results ensure that your legitimate emails reach their recipients without being mistakenly marked as spam. Tools like Suped provide insights into ARC validation.

What is the role of the 'cv' tag in an ARC-Seal header? - ARC - Email authentication - Knowledge base

An email envelope with an ARC-Seal, featuring a 'cv' tag, being examined by a magnifying glass.

The Authenticated Received Chain (ARC) is a crucial email authentication protocol designed to preserve authentication results across intermediate mail servers, such as mailing lists or forwarding services. Without ARC, these intermediaries can break standard email authentication methods like SPF and DKIM, potentially leading to legitimate emails being marked as spam.

Among the various components of ARC, the 'cv' tag within the ARC-Seal header plays a pivotal role in signaling the validation status of the ARC chain itself. This tag helps receiving email servers determine the trustworthiness of the authentication history presented in the email, even after multiple hops.

The basics of ARC and the 'cv' tag

Understanding the authenticated received chain

The Authenticated Received Chain (ARC) is an email authentication system designed by the Internet Engineering Task Force (IETF) to address the challenges of email forwarding. It allows intermediary mail servers to cryptographically sign the email's authentication results before forwarding it. This chain of signatures enables the final receiving server to verify the original authentication status, even if subsequent modifications would typically cause SPF or DKIM to fail.

Each ARC-Seal header contains several tags, each with a specific function. For instance, the 'i=' tag indicates the ARC instance, while the 's=' and 'd=' tags specify the signing domain and selector for DKIM verification, respectively. The 'cv' tag, however, is unique in its purpose: it explicitly states the validation outcome of the preceding ARC chain.

The 'cv' tag, short for 'chain validation', is present in each ARC-Seal header and indicates whether the previous ARC chain was successfully validated when this particular seal was added. It can have three primary values:

cv=none: No ARC chain was found, or it was not evaluated. This often happens on the first hop, where an initial ARC-Seal header is being generated.
cv=pass: The prior ARC chain successfully validated. This is the desired outcome, indicating that previous mail servers correctly handled the ARC process and the message's authentication history is intact.
cv=fail: The prior ARC chain validation failed. This suggests an issue with the ARC implementation by a previous intermediary or a potential tampering with the email.

The interpretation of the 'cv' tag is critical for the receiving mail server's decision-making process. A secure ARC chain (indicated by a series of 'cv=pass' tags) allows it to trust the historical authentication results, even if the message's headers or body were modified legitimately by an intermediary.

How the 'cv' tag works in practice

Chain validation in detail

When an email passes through an intermediary, like a mailing list or a forwarding service, that intermediary adds its own ARC-Seal header. Before doing so, it evaluates the existing ARC chain (if any) and records that outcome in the 'cv' tag of the *new* ARC-Seal. If the preceding ARC signatures are valid and intact, the 'cv' tag will be set to 'pass'.

Example of an ARC-Seal header with 'cv' tagtext

ARC-Seal: i=1; a=rsa-sha256; cv=none; s=arc; t=1678886400; d=example.com; bh=xyz; h=from:to:subject;
    b=ABCDEFG

A 'cv=fail' indicates that the validation of the prior ARC-Seal or ARC-Message-Signature headers failed. This could be due to malformed headers, incorrect signatures, or actual tampering. When a receiving mail server encounters a 'cv=fail' at any point in the chain, it signals that the integrity of the ARC chain has been compromised, reducing the trustworthiness of the message's authentication history.

A mail server evaluating an email's 'cv' tag, showing both a green checkmark for pass and a red 'X' for fail.

Understanding the 'cv' tag's outcome is crucial for diagnosing email deliverability issues. A consistent 'cv=pass' throughout an ARC chain reinforces the legitimacy of the sender, even when email headers are altered by forwarding. Conversely, a 'cv=fail' can trigger spam filters and lead to emails being quarantined or rejected, despite originating from a reputable source.

Impact on email deliverability and security

Ensuring trust with ARC

The primary benefit of the 'cv' tag and ARC in general is to maintain trust in email authentication. Mailbox providers like

Google and

Microsoft rely heavily on authentication protocols like SPF, DKIM, and DMARC to filter spam and protect users. However, legitimate forwarding services can inadvertently break these authentications. ARC, through its 'cv' tag, provides a mechanism to confirm the original authentication status, helping these messages reach the inbox.

Without ARC

DMARC failure: Emails often fail DMARC alignment after forwarding, leading to rejection or quarantine.
Increased spam score: Lack of verifiable authentication increases the likelihood of an email being flagged as spam.
Reduced deliverability: Legitimate emails may not reach the intended recipients' inboxes.

With ARC

Preserved authentication: The 'cv=pass' tag allows the DMARC results to be trusted, despite forwarding.
Improved deliverability: Higher inbox placement rates for emails sent through mailing lists and forwarding services.
Enhanced reputation: Consistent ARC validation contributes positively to sender reputation.

Monitoring the status of your ARC implementation, including the 'cv' tag, is a key part of maintaining email deliverability. Tools like Suped offer comprehensive DMARC monitoring, providing insights into your ARC headers and helping you identify any issues. Our platform's AI-powered recommendations can guide you in addressing problems and strengthening your email authentication posture.

Conclusion

The 'cv' tag: a critical component

The 'cv' tag in an ARC-Seal header is more than just a piece of technical data; it's a vital indicator of trust and integrity in the email forwarding process. By explicitly stating the validation status of the ARC chain, it allows receiving mail servers to make informed decisions about messages that might otherwise be incorrectly flagged as suspicious.

For organizations that rely on mailing lists or send emails through various intermediaries, ensuring that the 'cv' tag consistently reports 'pass' is essential for maintaining high email deliverability rates. It helps preserve your domain reputation and ensures your messages reliably reach their intended audience.

Utilizing Suped's DMARC monitoring platform provides clear visibility into ARC validation results, including the 'cv' tag, across all your email streams. This allows for proactive identification and resolution of authentication issues, ensuring your email ecosystem remains secure and effective.